A spectrometer is an optical apparatus for qualitatively and/or quantitatively analyzing materials of interest such as gases, liquids and solids according to a spectral signature characteristic of each material of interest. Correlation spectrometers provide for the analysis of materials of interest by processing polychromatic light from a source that has impressed thereon a spectral signature due to interaction of the light with the materials of interest and processing the light to generate alternately a first correlation with the spectral signature and a second correlation that is different from the first correlation.
A number of correlation spectrometers have been disclosed in the prior art, being either of a non-dispersive type in which the polychromatic light is selectively filtered or of a dispersive type in which the polychromatic light is dispersed for selective detection and analysis thereof.
For example, U.S. Pat. No. 3,811,776 to Blau discloses a non-dispersive correlation spectrometer that provides a first correlation between a particular gas of interest and a sample of the gas of interest, with a second correlation being derived from another pressure broadened sample of the gas of interest.
U.S. Pat. No. 3,955,891 to Knight et al discloses a dispersive correlation spectrometer having Hadamard-matrix spatial filters on the input and exit sides thereof, with a mechanical chopper provided to modulate particular wavelengths of light selected by the spatial filters for detection thereof.
U.S. Pat. No. 4,060,326 to Tirabassi et al discloses a dispersive correlation spectrometer for measuring concentrations of polluting gases on long and short geometric paths. The correlation spectrometer includes a comparison or reference cell containing a known concentration of the gas of interest for calibration of the spectrometer. The spectrometer includes a rotating correlation mask at an outlet focal plane of the spectrometer for sampling different wavelength ranges corresponding to different portions of the absorption spectrum of the gas of interest.
U.S. Pat. No. 4,193,691 to Fjarlie discloses a dispersive correlation spectrometer having a prism for dispersing the light to be analyzed, and a liquid-crystal-cell slit assembly for filtering the dispersed light by means of a pattern of slits formed therein that corresponds to a spectral signature of a material being analyzed.
U.S. Pat. No. 4,563,585 to Ward discloses a dispersive correlation spectrometer having a plurality of rotatable slits at the entrance to the spectrometer for alternately selecting between different wavelengths of light to be passed through the spectrometer.
The correlation spectrometers as described in the prior art are, for the most part, large and bulky. Furthermore, the prior art correlation spectrometers are, for the most part, tailored for the analysis of one or, in some instances, a very few materials of interest since some form of mechanical programming is incorporated into the instruments to provide the correlation, either in the form of gas cells which must be filled with a sample of a particular gas of interest or spatial filters (e.g. correlation masks) which must be photographically formed and precisely located. Thus, the prior art correlation spectrometers must be preprogrammed for the analysis of a very limited number of materials of interest; and are not easily reprogrammed for the analysis of other materials of interest, or for the analysis of a material whose composition is not known beforehand.
An advantage of the present invention is that an optical apparatus may be provided in the form of a correlation spectrometer having at least one diffraction grating programmed for analysis of one or more materials of interest. In some embodiments of the present invention, the diffraction grating may be electrically programmed for the analysis of a material of interest, and subsequently be electrically reprogrammed for the analysis of additional materials of interest. In other embodiments of the present invention, a plurality of diffraction gratings may be provided on a substrate and addressed either in sequence or in parallel for the analysis of many different materials of interest.
Another advantage of the present invention is that the analysis of a material of interest whose exact composition is not known beforehand may be performed with the correlation spectrometer by electrically programming the spectrometer for optically correlating a spectral signature from that material with an atlas containing information about many different materials either stored electronically for use in electrically programming a diffraction grating, or stored as an array of many different addressable diffraction gratings formed on a substrate.
A further advantage of the present invention is that the correlation spectrometer may include one or more diffraction gratings formed on a common substrate for alternately processing light at different wavelengths corresponding to a first spectrum that includes at least a portion of the spectral signature of a material of interest, and a second spectrum shifted in wavelength from the first spectrum.
Still another advantage of the present invention is that an optical apparatus may include one or more diffraction gratings having a plurality of grating elements in a multi-periodic spaced relationship for directing a plurality of wavelengths of light to a common point in space wherein a slit, aperture, optical fiber, or detector means may be located for forming a correlation spectrometer or an optical processor.
Still another advantage of the present invention is that the diffraction gratings may be formed on a substrate as an integrated device.
These and other advantages of the apparatus of the present invention will become evident to those skilled in the art.